This invention relates generally to electric motors and, more particularly, to a speed reduction switch for multiple speed induction motors.
Two speed induction motors are commonly used in swimming pool and spa pumps. Typically, induction motors include a magnetized rotor and a stator having a number of windings that form magnetic poles. When the magnetic poles of the stator are energized, the magnetic field in the windings creates a torque that causes the rotor to rotate. The magnetic field may be varied to change the speed of the motor. For example, a two pole/four pole motor has a high speed that corresponds to the energization of a "main" two pole winding, and a low speed that corresponds to the energization of an "auxiliary" four pole winding. The high and low speeds are selected as desired with an external switch.
Switching between the high and low speeds, however, can be problematic. The speed of the motor is inversely proportional to the number of stator poles, so the rotor of a four pole stator will generally rotate at about half the rate of a the rotor of a two pole stator when energized by the same power source. For example, a typical two pole/four pole motor in high speed rotates at about 3450 rpm, and rotates at about 1725 rpm in low speed. When the motor is switched from high speed to low speed, the rotor is abruptly braked to the low speed by the changed stator magnetic field. The rapid braking often causes unpleasant noise and vibration, which at least on some occasions leads to OEM (Original Equipment Manufacturers) rejection and field returns.
Accordingly, it would be desirable to provide a multiple speed motor that may be smoothly and quietly switched from higher speeds to lower speeds to prevent objectionable noise and vibration.